Tooth reconstruction



Patented Nov. 23, 1943 TOGTH RECONSTRUCTION Malcolm M. Benn-cw, Arlington. N. 1., assignor to E. L du Pont de Nemours & Company, Wilmington, 0121., a corporation of Delaware No Drawing. Application May 30, 1942, Serial No. 445,224

6 Claims.

This invention relates to tooth reconstructions and, more particularly, to dental inlays and crowns made of synthetic resins.

Material suitable for use in tooth reconstructions must be devoid of substances which are toxic to the mouth tissues or which irritate either 1 the tooth structure itself or the vita1 pulp within. It must be sufliciently hard to be free from flow under the pressure of reasonable usage at mouth temperatures, yet should be tough and nonfriable. Also, the materials should be substantially insoluble in water and other fluids, includ ing weak acids, which may be present in the mouth. The water absorptiveness should be low, for sanitation and permanence of fit, and the heat conductivity suillciently poor that the vital pulp of the tooth will not be in danger by extremes of heat or cold.

In addition to the above properties, material suitable for use for tooth reconstructions must also be capable of being easily and permanently cemented in place and must have a coefficient of thermal expansion similar to that of the tooth in order that the bond will not be weakened by temperature changes. 'The material must be adapted to give pleasing aesthetic properties in the finished product and, also, it is an advantage that the component ingredients going into the material should be easily manipulated.

Silver amalgam and gold, which have been used L.

extensively in the past in dental construction, have poor color and high heat conductivity. The former tends to discolor adjoining enamel and inclines to friability, while the latter is, in addition, expensive and diilicult to shape. Silicates are unsatisfactory because of solubility in the mouth juices and lack of strength.

More recently there have come into use synthetic resins,- of which the most suitable are those comprising polymerization products, particularly polymethyl methacrylate. These materials tend to lack the hardness requisite to satisfactory performance in dental crowns, inlays, and the like, however, and are not easily polished to a high gloss in the finishing operations ordinarily used by dentists. A further requirement of this type of material is that for it to becommercially practical it must be capable of being stored in an unfinished state for long periods of time without substantially changing in physical form or becoming otherwise unusable.

An object of the present invention is to provide a new and improved material for tooth reconstructions. A further object is to provide such a material of the synthetic resin type which has adequate shelf-life and may be used to form tooth reconstructions such as inlays and crowns having all the chemical, physical, physiological, and aesthetic properties required therein such as have been briefly reviewed above. Other objects will be apparent from the description of the invention given hereinafter.

The above objects are accomplished according to the present invention by making a tooth reconstruction from material essentially consisting of a mixture of methyl methacrylate and a relatively small proportion of methallyl methacrylate, the mixture being capable of conversion to a solid polymerized body of the desired shape by subjecting the mixture to polymerizing conditions.

More particularly, the invention comprises making a dental crown or inlay or'other tooth reconstruction from a material essentially consisting of a mixture of methyl methacrylate and 0.9%-5.0%. by weight of said mixture, of methallyl methacrylate, the mixture being composed of 80%50% of polymeric material and 20 of monomeric material.

In the preferred method of carrying out the invention there are provided as starting ingredients powdered or granular polymerized methyl methacrylate and, separately contained, monomeric methyl methacrylate with a small amount of monomeric methallyl methacrylate and containing a polymerization inhibitor. The polymer is mixed with the monomers, which are liquids miscible with each other, in such amounts and for such a period as to form a mass of doughy consistency which is then forced into a mold of the proper configuration and hardened under the influence of heat and pressure. The inlay, crown, or the like is then removed from the mold and aflixed in position on the tooth by means of a suitable cement. Alternatively, there may be used a monomer-polymer mixture containing a light-activated catalyst, such as benzoin; this mixture may be shaped in the mouth and there polymerized under the influence of visible or ultra-violet radiation as disclosed in the copending application of F. C. Hahn, Serial No. 445,205, filed of even date herewith.

The following examples in which all proportions are given by weight unless otherwise stated, illustrate specific embodiments of the invention:

Example I and containing a small amount of residual a screen having 55 x meshes per inch benz oyl peroxid'ekatalyst and, separately contained, a liquid consisting of:

Parts Methyl methacrylate monomer.-. 30.4 Methallyl methacrylate monomer... 1.6 Hydroquinone 0.009

- boiling water for thirty minutes, after whichit is removed and opened. The inlay of solid polymer thus formed is adhered in position on the tooth by means of oxyphosphate of zinc cement and subsequently polished. It is hard and tough, does not chip in normal usage, has no taste, and remains firmly cemented for an indefinite period of time.

Example II A blended Jacket crown is prepared by the use of material similar to that used in Example I but of two slightly diiferent tints. Two molds are required, one having a cavity the size of the desired finished crown and the other a smaller cavity.- Dough of one color is pressed into the smaller mold which is then closed and immersed in boiling water for five minutes, after which the toughened dough is removed and invested in a dough of the second color in the larger mold. Curing is efiected as in Example I. The completed jacket crown is fastened to the tooth by means of a cement. After having been cleaned and polished, the crown has a color and a lustre closely resembling that of natural enamel.

Example III Dry polymerized methyl methacrylate powder containing residual catalyst; is placed in an inlay mold of the proper configuration. To this is added, drop by drop, a liquid mixture consisting of:

Parts Methyl methacrylate monomer 27.2 Methallyl methacrylate monomer 2.8 Hydroquinnno 0,008

Addition ofthe liquid is stopped as soon as the powder is completely saturated. The mold is completely closed and treated in accordance with '70 parts of powdered polymerized methyl methacrylate of the desired color are blended with a liquid consisting of:

Parts Methyl methacrylate monomer 29.0 Methallyl methacrylate monomer 1.0

Example V 62 parts of finely divided powder consisting of a copolymer of methyl methacrylate and meth-' allyl methacrylate in the ratio 96:4, are mixed with 38 parts of a liquid mixture of methyl methacrylate monomer and methallyl methacrylate monomer in the ratio :1. The resulting dough is used in forming an inla in accordance with Example I. The inlay is hard, tough and easily polished.

Example VI 65 parts of finely divided powder consisting of a copolymer of methyl methacrylate and methallyl methacrylate in the ratio of 95:5 are 'mixed with 35 parts of methyl methacrylate monomer. Inlays made from the resulting dough have satisfactory hardness and toughness and can be given a polish resembling that of natural enamel.

The above examples are merely illustrative and the invention broadly comprises, in making tooth reconstructions, the use of a mixtur of methyl methacrylate and a minor proportion of methallyl methacrylate, the mixture being capable of conversion to a solid polymerized body I by subjecting it to polymerizing conditions.

in a flask under an atmosphere of oxygen-free v nitrogen. The dough is kept free from exposure to air until it has attained the proper consistency and is no longer sticky. It is then forced into a mold which is thereupon tightly closed and immersed in boiling water for twenty minutes.. The resulting dental part is found to be as hard as that prepared in accordance with Example I.

Normally a mixture of powdered polymer and liquidmonomer will be used in carrying out this invention and it is preferred that the powder be finely divided enough to pass a 55 x mesh screen. The ratio of monomer to polymer may be varied but usually will be selected to provide a doughy mass of kneadable consistency; the use of 32 parts of monomer to 68 parts of polymer is satisfactory although the weight ratio of polymer to monomer may well be varied from about80/20 to 50/50. Thicker doughs are difficult to manipulate and tend to yield friable solid bodies while thinner mixtures tend to shrink excessively and to bubble in setting up.

While not a preferred method of carrying out the invention, a dental inlay or the like may be made according to the present invention by casting in a mold a substantially unpolymerized mix- The amount of methallyl methacrylate mono men used will ordinarily not be less than about 0.9% nor more than about 3.0% by weight of the total mixture. When less than the smaller amount is employed, the resulting resin is not hard enough while an excess over the larger fig-- ure results in increased brittleness. A resin formed by polymerizing a mixture containing about 1.5% methallyl methacrylate is preferred for crowns and inlays.

While it usually will be more convenient to supply the methallyl methacrylate as monomer, it may be used as a preformed copolymer with methyl methacrylate. With the copolymer there is employed monomeric methyl methacrylate or a mixture thereof with monomeric methallyl methacrylate. However, the amount of methallyl methacrylate monomer should not exceed about 3% by weight of the mixture nor should the total amount of methallyl methacrylate, as monomer and as a, constituent of the copolymer, exceed about 5%, nor be less than about 0.9%.

The material of the present invention essentially consists of methyl methacrylate and methallyl methacrylate. Nevertheless, other acrylic and methacrylic esters'as well as interpolymers with one. another or with other polymerizable substances may be used to replace minor proportions of the methyl methacrylate to develop specific characteristics desiredin the finished product. Such minor modification of the methyl methacrylate component of the material comes within the scope of the invention. Obviously, suitable pigments, dyes, or other modifiers may be included; the polymerized resin is transparent and colorless and this greatly facilitates obtaining desired aesthetic eifects by incorporating pigments and dyes in the composition.

The polymer component of the material will ordinarily contain suflicient residual catalyst to complete the polymerization of the dough but further catalyst may be added. Hydroquinone in an amount of about 0.025%, by weight of the monomer, has been found a satisfactory storage inhibitor for the liquid monomer although the kind and quantity of inhibitor need depend only upon satisfactory performance.

The mold in which the tooth restoration is to be formed, will usually be made by first taking a wax impression of the prepared tooth cavity, investing the impression in plaster of Paris and dissolving out the wax in a manner well known to those skilled in the art.

Hardening of the material forming the tooth restoration is accomplished by subjecting it to polymerizing conditions, usually effected by the influence of heatand carried out under pressure. Temperatures of 30 C. to 105 C. and pressures of 150 to 250 pounds per square inch are suflicient but'the invention is not limited to these ranges. Heat in the absence of appreciable pressure can be used in forming a satisfactory product.

Oxyphosphate of zinc, a well known cement to the dental profession, is' suitable for cementing inlays and crowns made of the material herein considered, to teeth. Other cements such as zinc The polymerized product of this invention is I hard, tough, and physiologically inactive. It is insoluble in mouth fluids, is a poor heat conductor, has a coefficient of thermal expansion reasonably close to that of natural teeth, and may be readily cemented in position on the tooth.

The aesthetic properties of this material makes it especially desirable for use in the anterior regions of the mouth. Its ability to be delicately pigmented is coupled with permanence of color and complete freedom from any tendency to discolor the natural enamel.

An outstanding advantage of the herein considered material is its ability to develop a high polish under the action of a mechanical bufier; Unmodified polymerized methyl methacrylate and many copolymers therewith become soft and rubbery with the heat generated by a power buffer and the resulting finish is so dull as to be easily distinguishable from that of natural enamel. An important practical advantage of this material is its outstanding ability to be stored in convenient form for long periods of time with the aid of a common, inexpensive inhibitor such as hydroquinone.

As many apparently widely difierent embodiments. of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the inve'ntion is not limited to the specific embodiments thereof except as defined in the appended claims.

. I claim:

l. A tooth reconstruction essentially consisting of a polymerized mixture of methyl methacrylate and 0.9%-5.0%, by weight of said mixture, of methallyl methacrylate.

.2. A tooth reconstruction essentially consisting of a polymerized mixture of. methyl methacrylate and about 1.5%, by weight of said mixture, of methallyl methacrylate.

3. Material for tooth reconstructions essentially consisting of a mixture of methy1 methacrylate and 0.9%-5.0%, by weight of said mixture, of methallyl methacrylate, said mixture being capable of conversion to a solid polymerized body of the desired shape by subjecting same to polymerizing conditions.

4. Material for tooth reconstructions essentially consisting of a mixture of methyl methacrylate and 0.9%5.0%, by weight of said mixture, of methallyl methacrylate, said mixtur being composed of %-50% of polymeric material and 20%-50% of monomeric material. I

5. Material for tooth reconstructions essentially consisting of a mixture of 80%50% of polymerized methyl methacrylate and 20%50% of monomeric methyl methacrylate and monomeric methallyl methacrylate, said methallyl methacrylate amounting to 0.9%-3.0%, by weight of the mixture.

MALCOLM M. RENFREW. 

